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The greenhouse effect is a natural phenomenon whereby heat-trapping gases in the atmosphere, primarily water vapor, keep the Earth’s surface warm. Human activities, primarily burning fossil fuels and changing land cover patterns, are increasing the concentrations of some of these gases, amplifying the natural greenhouse effect.
Click on image for full size Source: Modified from the Marian Koshland Science Museum of the National Academy of Sciences’ “Global Warming: Facts & Our Future” 2004

Fundamental Concept 1b.
When Earth emits the same amount of energy as it absorbs, its energy budget is in balance, and its average temperature remains stable.

Fundamental Concept 1c.
The tilt of Earth’s axis relative to its orbit around the Sun results in predictable changes in the duration of daylight and the amount of sunlight received at any latitude throughout a year. These changes cause the annual cycle of seasons and associated temperature changes.

Fundamental Concept 1e.
A significant increase or decrease in the Sun’s energy output would cause Earth to warm or cool. Satellite measurements taken over the past 30 years show that the Sun’s energy output has changed only slightly and in both directions. These changes in the Sun’s energy are thought to be too small to be the cause of the recent warming observed on Earth.

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